QGIS API Documentation  3.25.0-Master (dec16ba68b)
qgsscalecalculator.cpp
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1 /***************************************************************************
2  qgsscalecalculator.h
3  Calculates scale based on map extent and units
4  -------------------
5  begin : May 18, 2004
6  copyright : (C) 2004 by Gary E.Sherman
7  email : sherman at mrcc.com
8  ***************************************************************************/
9 
10 /***************************************************************************
11  * *
12  * This program is free software; you can redistribute it and/or modify *
13  * it under the terms of the GNU General Public License as published by *
14  * the Free Software Foundation; either version 2 of the License, or *
15  * (at your option) any later version. *
16  * *
17  ***************************************************************************/
18 
19 #include <cmath>
20 #include "qgslogger.h"
21 #include "qgsrectangle.h"
22 #include "qgsscalecalculator.h"
23 
25  : mDpi( dpi )
26  , mMapUnits( mapUnits )
27 {}
28 
29 void QgsScaleCalculator::setDpi( double dpi )
30 {
31  mDpi = dpi;
32 }
34 {
35  return mDpi;
36 }
37 
39 {
40  QgsDebugMsgLevel( QStringLiteral( "Map units set to %1" ).arg( QString::number( mapUnits ) ), 3 );
41  mMapUnits = mapUnits;
42 }
43 
45 {
46  QgsDebugMsgLevel( QStringLiteral( "Map units returned as %1" ).arg( QString::number( mMapUnits ) ), 4 );
47  return mMapUnits;
48 }
49 
50 double QgsScaleCalculator::calculate( const QgsRectangle &mapExtent, double canvasWidth ) const
51 {
52  if ( qgsDoubleNear( canvasWidth, 0. ) || qgsDoubleNear( mDpi, 0.0 ) )
53  {
54  QgsDebugMsg( QStringLiteral( "Can't calculate scale from the input values" ) );
55  return 0;
56  }
57 
58  double conversionFactor = 0;
59  double delta = 0;
60  calculateMetrics( mapExtent, delta, conversionFactor );
61 
62  const double scale = ( delta * conversionFactor ) / ( static_cast< double >( canvasWidth ) / mDpi );
63  QgsDebugMsgLevel( QStringLiteral( "scale = %1 conversionFactor = %2" ).arg( scale ).arg( conversionFactor ), 4 );
64  return scale;
65 }
66 
67 QSizeF QgsScaleCalculator::calculateImageSize( const QgsRectangle &mapExtent, double scale ) const
68 {
69  if ( qgsDoubleNear( scale, 0.0 ) || qgsDoubleNear( mDpi, 0.0 ) )
70  {
71  QgsDebugMsg( QStringLiteral( "Can't calculate image size from the input values" ) );
72  return QSizeF();
73  }
74  double conversionFactor = 0;
75  double delta = 0;
76 
77  calculateMetrics( mapExtent, delta, conversionFactor );
78  const double imageWidth = ( delta * conversionFactor ) / ( static_cast< double >( scale ) ) * mDpi;
79  const double deltaHeight = ( mapExtent.yMaximum() - mapExtent.yMinimum() ) * delta / ( mapExtent.xMaximum() - mapExtent.xMinimum() );
80  const double imageHeight = ( deltaHeight * conversionFactor ) / ( static_cast< double >( scale ) ) * mDpi;
81 
82  QgsDebugMsgLevel( QStringLiteral( "imageWidth = %1 imageHeight = %2 conversionFactor = %3" )
83  .arg( imageWidth ).arg( imageHeight ).arg( conversionFactor ), 4 );
84 
85  return QSizeF( imageWidth, imageHeight );
86 }
87 
88 void QgsScaleCalculator::calculateMetrics( const QgsRectangle &mapExtent, double &delta, double &conversionFactor ) const
89 {
90  delta = mapExtent.xMaximum() - mapExtent.xMinimum();
91  switch ( mMapUnits )
92  {
94  // convert meters to inches
95  conversionFactor = 39.3700787;
96  break;
98  conversionFactor = 12.0;
99  break;
101  // convert nautical miles to inches
102  conversionFactor = 72913.4;
103  break;
104  default:
106  // degrees require conversion to meters first
107  conversionFactor = 39.3700787;
108  delta = calculateGeographicDistance( mapExtent );
109  break;
110  }
111 }
112 
114 {
115  // need to calculate the x distance in meters
116  // We'll use the middle latitude for the calculation
117  // Note this is an approximation (although very close) but calculating scale
118  // for geographic data over large extents is quasi-meaningless
119 
120  // The distance between two points on a sphere can be estimated
121  // using the Haversine formula. This gives the shortest distance
122  // between two points on the sphere. However, what we're after is
123  // the distance from the left of the given extent and the right of
124  // it. This is not necessarily the shortest distance between two
125  // points on a sphere.
126  //
127  // The code below uses the Haversine formula, but with some changes
128  // to cope with the above problem, and also to deal with the extent
129  // possibly extending beyond +/-180 degrees:
130  //
131  // - Use the Halversine formula to calculate the distance from -90 to
132  // +90 degrees at the mean latitude.
133  // - Scale this distance by the number of degrees between
134  // mapExtent.xMinimum() and mapExtent.xMaximum();
135  // - For a slight improvemnt, allow for the ellipsoid shape of earth.
136 
137 
138  // For a longitude change of 180 degrees
139  const double lat = ( mapExtent.yMaximum() + mapExtent.yMinimum() ) * 0.5;
140  static const double RADS = ( 4.0 * std::atan( 1.0 ) ) / 180.0;
141  const double a = std::pow( std::cos( lat * RADS ), 2 );
142  const double c = 2.0 * std::atan2( std::sqrt( a ), std::sqrt( 1.0 - a ) );
143  static const double RA = 6378000; // [m]
144  // The eccentricity. This comes from sqrt(1.0 - rb*rb/(ra*ra)) with rb set
145  // to 6357000 m.
146  static const double E = 0.0810820288;
147  const double radius = RA * ( 1.0 - E * E ) /
148  std::pow( 1.0 - E * E * std::sin( lat * RADS ) * std::sin( lat * RADS ), 1.5 );
149  const double meters = ( mapExtent.xMaximum() - mapExtent.xMinimum() ) / 180.0 * radius * c;
150 
151  QgsDebugMsgLevel( "Distance across map extent (m): " + QString::number( meters ), 4 );
152 
153  return meters;
154 }
A rectangle specified with double values.
Definition: qgsrectangle.h:42
double yMaximum() const SIP_HOLDGIL
Returns the y maximum value (top side of rectangle).
Definition: qgsrectangle.h:193
double xMaximum() const SIP_HOLDGIL
Returns the x maximum value (right side of rectangle).
Definition: qgsrectangle.h:183
double xMinimum() const SIP_HOLDGIL
Returns the x minimum value (left side of rectangle).
Definition: qgsrectangle.h:188
double yMinimum() const SIP_HOLDGIL
Returns the y minimum value (bottom side of rectangle).
Definition: qgsrectangle.h:198
double calculate(const QgsRectangle &mapExtent, double canvasWidth) const
Calculate the scale denominator.
void setDpi(double dpi)
Sets the dpi (dots per inch) for the output resolution, to be used in scale calculations.
void setMapUnits(QgsUnitTypes::DistanceUnit mapUnits)
Set the map units.
double calculateGeographicDistance(const QgsRectangle &mapExtent) const
Calculate the distance between two points in geographic coordinates.
QgsUnitTypes::DistanceUnit mapUnits() const
Returns current map units.
QSizeF calculateImageSize(const QgsRectangle &mapExtent, double scale) const
Calculate the image size in pixel (physical) units.
double dpi() const
Returns the DPI (dots per inch) used in scale calculations.
QgsScaleCalculator(double dpi=0, QgsUnitTypes::DistanceUnit mapUnits=QgsUnitTypes::DistanceMeters)
Constructor.
DistanceUnit
Units of distance.
Definition: qgsunittypes.h:68
@ DistanceMeters
Meters.
Definition: qgsunittypes.h:69
@ DistanceDegrees
Degrees, for planar geographic CRS distance measurements.
Definition: qgsunittypes.h:75
@ DistanceFeet
Imperial feet.
Definition: qgsunittypes.h:71
@ DistanceNauticalMiles
Nautical miles.
Definition: qgsunittypes.h:72
As part of the API refactoring and improvements which landed in the Processing API was substantially reworked from the x version This was done in order to allow much of the underlying Processing framework to be ported into c
bool qgsDoubleNear(double a, double b, double epsilon=4 *std::numeric_limits< double >::epsilon())
Compare two doubles (but allow some difference)
Definition: qgis.h:1990
#define QgsDebugMsgLevel(str, level)
Definition: qgslogger.h:39
#define QgsDebugMsg(str)
Definition: qgslogger.h:38